Septic Design for a Bedroom Addition in Central NY
Adding bedrooms, finishing a basement or rebuilding the house? A licensed PE checks the tank and field against the new bedroom count, and Backwell handles the upgrade if it falls short.
Bedrooms. Gallons. Permits.
How an added bedroom changes what your septic system has to handle.
Under NYS DOH Fact Sheet #1, a licensed design professional must evaluate the existing septic system and submit that evaluation when a home alteration increases the number of bedrooms. The evaluation documents whether the system meets state and local standards, is in good condition, and can handle the added flow.
Appendix 75-A sets 110 gallons per day per bedroom for new construction. Table 1 lists 130 gallons per day per bedroom for houses with pre-1994 fixtures and 150 for pre-1980 fixtures. The handbook says expansions for added rooms that will or can be used as bedrooms should also follow Table 1.
Appendix 75-A Table 3 sets a minimum of 1,000 gallons for 1 to 3 bedrooms, 1,250 gallons for 4, 1,500 for 5 and 1,750 for 6. Add 250 gallons for each bedroom over six. A garbage grinder counts as an extra bedroom for tank size, and the handbook says a whirlpool bath or spa connected to the plumbing should too.
The NYS DOH handbook says expansion attics, basements, sleeping porches, dens and recreation rooms that may be converted to permanent bedrooms in the future should be considered when calculating design flow. The handbook also suggests reviewing architectural plans for potential bedrooms.
In some places it does. Cayuga County sends a memo to the town code enforcement officer after it accepts the septic plan so the town can then consider the building permit. In the Skaneateles Lake watershed, no addition or change of use may be undertaken until the health department approves the septic work or accepts the existing system in writing.
Fact Sheet #1 says that if the existing system does not meet design standards or needs significant changes, the licensed design professional prepares plans and oversees the installation of the alterations. That can mean a bigger tank, more trench, or a new field.
It depends on the perc rate. From Appendix 75-A trench tables at 110 gallons per day per bedroom, going from 3 to 4 bedrooms on soil that percs at 21 to 30 minutes per inch takes the trench from 275 feet to 367 feet, an extra 92 feet.
Septic design for a bedroom addition
In New York, septic systems are sized by bedrooms, not by bathrooms or square feet. Add a bedroom and the system's required capacity goes up, even if the same people live there. That is why a bedroom addition, a finished basement with a sleeping room, or a full teardown and rebuild triggers a septic capacity check before the building permit or certificate of occupancy moves forward in many Central New York towns.
The short answer: a licensed design professional checks whether the existing tank and field meet today's sizing for the new bedroom count. If they do, the evaluation goes in. If not, a stamped design for the upgrade goes in.
The state rule on additions
NYS DOH Fact Sheet #1, "Need for Licensed Design Professionals," prepared with the State Education Department, says an evaluation must be done and submitted by a licensed design professional for:
- home alterations that increase the number of bedrooms,
- complete home replacements, even with the same number of bedrooms, and
- alterations that significantly increase wastewater.
The evaluation has to document three things: whether the existing system complies with state and local design standards, whether it is in good condition and working, and whether it can properly treat the added flow. If it falls short, the design professional prepares plans and oversees the upgrade.
Gallons per bedroom under Appendix 75-A
Appendix 75-A, section 75-A.3, sets design flow by bedroom count and fixture age:
| Plumbing fixtures | Design flow per bedroom |
|---|---|
| Post-1994 (1.6 gal/flush toilets, 2.5 gpm faucets and showerheads) | 110 gallons per day |
| Pre-1994 (3.5 gal/flush toilets, 3.0 gpm) | 130 gallons per day |
| Pre-1980 (over 3.5 gal/flush, over 3.0 gpm) | 150 gallons per day |
| Waterless toilets (graywater only) | 75 gallons per day |
New construction is designed at 110. The handbook says design flow for expanding a system to meet an actual or potential occupancy increase, such as adding rooms that will or can be used as bedrooms, should also follow this table. Low flow fixtures do not shrink the tank: the handbook says the biological load stays about the same, so tank size is not reduced for water saving fixtures.
Tank size by bedroom count
Appendix 75-A Table 3 minimum septic tank sizes:
| Bedrooms | Minimum tank capacity | Minimum liquid surface area |
|---|---|---|
| 1, 2 or 3 | 1,000 gallons | 27 sq ft |
| 4 | 1,250 gallons | 34 sq ft |
| 5 | 1,500 gallons | 40 sq ft |
| 6 | 1,750 gallons | 47 sq ft |
Over six bedrooms, add 250 gallons and 7 square feet per bedroom. A garbage grinder counts as one more bedroom for tank sizing. The handbook says a whirlpool bath or spa connected to the house plumbing should too. So a 3 bedroom house adding a fourth bedroom moves from a 1,000 gallon minimum to 1,250 gallons. Add a garbage grinder and it is sized like a 5 bedroom house at 1,500 gallons.
How much more field an extra bedroom takes
Trench length scales with flow and perc rate. These figures come from the Appendix 75-A conventional trench table (2 foot wide trench, 110 gallons per day per bedroom):
| Perc rate (min/in) | 3 bedrooms (330 gpd) | 4 bedrooms (440 gpd) | 5 bedrooms (550 gpd) |
|---|---|---|---|
| 1 to 5 | 138 ft | 184 ft | 230 ft |
| 11 to 15 | 207 ft | 275 ft | 344 ft |
| 21 to 30 | 275 ft | 367 ft | 459 ft |
| 46 to 60 | 367 ft | 489 ft | 612 ft |
On a slow soil, one more bedroom can mean over 100 feet of new trench, plus room for the 50 percent reserve area Appendix 75-A asks for where possible. Once total trench length reaches 500 feet, the table requires dosing. That is where a lot that worked fine for 3 bedrooms can run out of room for 5, and the design moves toward pressure distribution, a raised bed or a mound.
Rooms that count as future bedrooms
The handbook says expansion attics, basements, sleeping porches, dens and recreation rooms that may be turned into permanent bedrooms should be considered when setting design flow. It suggests a homeowner interview and a review of the architectural plans for potential bedrooms, high water use fixtures (multi-head showers, whirlpool tubs) and home businesses such as hair styling, daycare or catering.
How the building permit ties in, county by county
- Cayuga County. The county sanitary code says no one may expand, enlarge or convert a structure or system in a way that will or may increase wastewater without county approval and permits. After the county accepts a septic plan, it sends a memo to the town code enforcement officer so the town may consider issuing the building permit. Fees: $150 for a new system review, $100 or $150 for modifying an existing one.
- Cortland County. The sanitary code says no one may enlarge or extend a building in a way that changes, or may change, the volume or character of wastewater without first getting a permit. The 2026 fee for a sewage disposal permit is $200.
- Skaneateles Lake watershed (Towns of Skaneateles, Spafford and Marcellus in Onondaga County, plus parts of Cayuga and Cortland counties). 10 NYCRR 131.1 says no new construction, alteration, addition or change of use (such as turning a seasonal camp into a year round home) may start until the health department has approved the septic work or issued a written notice accepting the existing system. The City of Syracuse, as the water supplier, is notified and reviews.
- Onondaga County. The county's repair application is valid only where there is no change in total usage of the property, so an addition that adds bedrooms goes through plan review ($310 for systems under 1,000 gallons per day in 2026). For raised systems, the county's design guidelines say it will not authorize release of the building permit until the fill is in place and certified by the design professional.
- Oswego County. Approval letters for septic plans go to both the owner and the town or city code enforcement officer.
- Madison County. The Town of Madison permit application says applications that include a new septic system must show it on the plot diagram.
Problem and solution
Old system, no records. Many Central New York houses have fields nobody has seen since they went in. The evaluation starts with finding and opening the tank, locating the distribution box and field, and checking the tank's size against Table 3.
Tank too small, field fine. A larger or second tank can bring the system up to the new bedroom count while the field stays.
Field too short. New trench is added where the soil and setbacks allow, tested first with a perc test and deep hole.
No room left. Small lots with a well nearby may not fit more conventional trench inside the Table 2 setbacks (100 feet from a well to the absorption field). See small lot setbacks and enhanced treatment design.
Camp turning year round. See seasonal camp to year round.
Timeline
Put the septic check at the front of the addition, before the architect finalizes the plan. If the evaluation shows the system is big enough, it goes in with the building permit. If an upgrade is needed, add soil testing, design and county review. Cayuga County says plan approval may take up to 4 to 6 weeks. The full sequence is on the septic engineering process page, and fees are on the cost page.
Designs are prepared and stamped by a New York licensed Professional Engineer (PE). Backwell coordinates the design, the health department submittal, and the install.
Sources
- NYS DOH, Residential Onsite Wastewater Treatment Systems Design Handbook (includes Appendix 75-A and Fact Sheet #1)
- NYS DOH, 10 NYCRR Appendix 75-A, Wastewater Treatment Standards: Residential Onsite Systems
- Cayuga County Sanitary Code
- Cayuga County Health Department, Instructions for New Construction
- Cayuga County, Application to Install a New Septic System (rev. 1/7/2025)
- Cayuga County, Application to Modify/Replace an Existing Septic System (rev. 1/7/2025)
- Cortland County Sanitary Code (April 1, 2026)
- Cortland County Health Department, Fee Schedule effective April 1, 2026
- 10 NYCRR 131.1, Skaneateles Lake watershed rules and regulations
- Onondaga County, Application for Repair of Sewage Disposal System
- Onondaga County Local Law 11 of 2025, Health Department fees (effective 1/1/2026)
- Onondaga County Health Department, Design Guidelines for Individual Sewage Systems
- Oswego County Health Department, Septic Plan Review
- Town of Madison, building permit application
